This project is aimed at the development of an enzyme immunoassay (ELISA) for screening blood for antibodies to Babesia microti. Human babesiosis is a malaria-like illness due to infection of red blood cells by various species of protozoan parasites belonging to the genus Babesia. Most human infections reported in the United States are caused by B. microti which is endemic in parts of the northeast and north central United States. B. microti infections are often asymptomatic but can also progress to severe illness and even death, particularly in individuals who are immunosuppressed, asplenic or elderly. Recent cases of fatal transfusion-transmitted babesiosis have led to the identification of this pathogen as a significant threat to the blood supply. Currently no commercial, validated and FDA approved test are available for B. microti, and donor questionnaires have proven ineffective in preventing introduction of Babesia into the blood supply. Tests now in use including immunofluorescence, microscopy and PCR are not easily adaptable to high throughput blood screening. This proposal describes the development and clinical trial of a microplate-based Babesia ELISA which can be performed on an automated, high throughput platform, with the sensitivity and specificity required for blood screening. The research plan has five specific aims: 1) completion of assay development, proceeding from an optimized R&D prototype to a manufacturable kit, 2) manufacturing scale up, 3) evaluation of assay performance on blood donors from Babesia endemic and non-endemic areas in an IND clinical trial, 4) follow-up study of seropositive donors to inform donor deferral and re-entry policy, and 5) preparation and submission of a BLA application to the FDA for licensure for blood screening use.